There are many applications in which high bandwidth communication is required over a transmission line with a device that may be moveable relative to the transmission line.
One example is a communications network in an environment such as a hospital where imaging equipment needs to transmit or receive high volumes of data at various locations in the hospital. It can be difficult providing appropriate physical connections between the equipment and the transmission line. This problem is compounded if the equipment needs to be moved for use around a variety of physical locations.
Another example is communication with pick-up devices supplied with power from an energised track, such as an HID/IPT (High Efficiency Inductive Power Distribution/ Inductive Power Transfer). HID/IPT systems are very popular for many practical applications. They can work in very harsh environments, as they transfer power without physical contact and are therefore tolerant of environmental hazards such as water, acids, dirt and grime. Yet they themselves produce no harmful residues.
In consequence HID/IPT systems can operate in factories where they provide high reliability and immunity to paint and fumes. They can also operate in Clean Rooms where the level of cleanliness is very high and the HID/IPT system is compatible.
The ability for communication with a device powered by an HID/IPT system is becoming increasingly important. HID/IPT systems usually transfer power to devices that have a task to perform, for example the devices may be carriages which perform automated processes or which are required to travel to a selected location. The tasks that the devices are to perform can be automated to a greater degree and made far more efficient by providing a means of communication between devices and/or between each device and a system control module.
A communications system for an HID/IPT system must share the same advantages as the HID/IPT system i.e. it must transfer information without physical contact and must be tolerant of a harsh environment yet produce no residues, or electromagnetic interference, itself.
HID/IPT systems operate in a wide range of environments where the power cables of the primary conductive path or track may be in air, or water, or even concrete. In these special circumstances it is unlikely that one particular type of communications system will be universally applicable.
In U.S. Pat. No. 6,005,475 a communications system where the HID/IPT track is tuned to two frequencies at the same time has been disclosed. This system has the advantages of low cost (as no additional conductors are required) and applicability, as wherever there is power there are communication signals as well. But the range of applicability of this technique is limited as the bandwidth that is available using pick-ups tuned to both a power frequency and a communications frequency at the same time is limited. In practice bandwidths of less than 50 kHz are to be expected. The method has the advantage that it is operable with all HID/IPT media, for example wood, concrete, water, and air. However, in many circumstances its bandwidth is simply too small.
Wider bandwidth communication systems use microwaves, for example adhering to standards such as IEEE 802.11a or b, but these bands are becoming congested and microwave is not acceptable by many potential users of HID/IPT systems. In factory conditions microwaves can also suffer from shadowing. This means that extra diversity must be introduced which adds to spectral clutter. Wideband systems can also use leaky feeders. These are essentially distributed antennas and radiate widely making compliance with emission standards difficult when wide bandwidths are needed. Leaky feeders are also very expensive. Other communications systems economise by using the HID/IPT cables to propagate radio signals as well and having antennas distributed around the track to receive the signals. These systems are effective but leak radiation as power wires are not good RF conductors, and consequently they have a restricted bandwidth and range.
In another system, which is disclosed in International Application WO03005380, a data cable runs alongside the primary power cable of an HID/IPT system. The conductors of the data cable are arranged in such a way as to keep coupling between the data cable and the power cable to a minimum, and therefore reduce “crosstalk”. The frequency of the information signal in the data cable is typically less than 1 Megahertz and the transmission rate is about 10 to 150 kbit/s. This system requires a very specific geometric relationship between the data cable and the power cable.